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Home , Harrie Massey, Hughes Medal, James Chadwick, Lawrence Bragg, Nevill Francis Mott

84 PROFILE head of theoretical at the Cavendish, Ralph Fowler, he set about finding his own problem which he decided Professor Sir should be on the application of wave CH, KB, ScD, FRS, Chevalier Ordre Nat. du Merite, Nobel Laureate in Physics mechanics to the scattering of charged particles - an active area of experimental investigation led by . This work was published in the Proceedings of the Royal Society. One year after he graduated with first- in numerous countries - via handwritten class honours in 1928, Fowler arranged a letters most of which contained gems of grant to enable him to spend a year ideas that were treasured by their abroad, which he divided between visits to recipients. Unlike most of us he didn’t and Gottingen. At Copen­ have a cluttered desk with a massive pile in hagen he discussed with the the in-tray; he worked in “real time”, startling prediction of Dirac in writing first drafts of papers as ideas that the electron carried a , and began occurred to him and answering letters, work on a related problem. He clearly responding to requests for information, found it exciting to be in the company of and writing referee’s reports, references Bohr and other famous scientists of the era etc., often by return post. His mind was - for example, Pauli, Gamow, Heisenberg his principal filing cabinet. In these ways and Hartree, who visited Bohr’s famous he drew together ideas and people from institute. Before moving on to Gottingen, different disciplines in a unique manner. he returned for a few months to Cam­ bridge where he solved an important Born 30 September 1905, died 8 August 1996 problem relating to the scattering of alpha Early years particles. Chadwick, Rutherford’s second The death of Sir Nevill Mott at age 90 It was natural that Mott should take an in command, was clearly impressed and brought to a close the illustrious “life in early interest in physics. His mother and took Mott along to see Rutherford who ” of a man who stood academically, father both worked at the Cavendish exclaimed “If you think of anything else as well as literally, head and shoulders Laboratory at Cambridge University with like this, come and tell me.” This was above his contemporaries, many of whom J.J. Thomson, a few years after J.J.’s praise indeed and he later recalled it was were awed and inspired by his remarkable identification of cathode rays with on that day when he gained complete powers of intuition, his penetrating insight “corpuscles” (electrons) in 1897. In his confidence in his ability to make a career and his versatile creativity. autobiography “A Life in Science” he in theoretical physics. The staggeringly long length of his records that quite early in his life his While in Gottingen, where he had scientific career - his first paper was parents communicated to him the hoped to work with - an published in 1927 during the early days of excitement and importance of physics. opportunity that did not materialise owing quantum theory and his last on high- His mother taught him at home until he to Born being in poor health - Mott temperature superconductivity in 1996, was ten when he became a weekly boarder received an offer of a lectureship at four months after his death - must be at a small preparatory school near University under W.L. Bragg, unprecedented, and the range of his Stafford. Before he left that school at age which he accepted. A course of lectures he contributions to physics possibly 13 he was, according to his own words, gave there on wave mechanics led to his unequalled. He was the author of at least advanced in algebra and Latin and had first book “An Outline of Wave Mechanics” fifteen books and over three hundred been introduced to calculus. He spent the written at age 25. It was at Manchester scientific papers, covering a range of next five years as a boarder at Bristol’s where his interest turned to the properties topics as diverse as wave mechanics, , where a gifted teacher, H. of materials, stimulated no doubt by the , the properties of , C. Beaven, instilled in him the beauty of power of the new technique of X-ray ionic crystals, and glasses, mathematics. Later he recalled the to determine the structure and the of superconductivity - an excitement he felt when, at about age 16, of - a method which earned W.L. impressive list but, even so, one which he realised why any number raised to the Bragg (later Sir Lawrence and Cavendish does not convey or do justice to the range power nought equalled unity! In Professor of ) the of his activities. December 1923 he was awarded a major for physics, jointly with his Nevill Mott was a theoretical scholarship by St John’s College, father W.H. Bragg. - a label far too narrow and restrictive to Cambridge, where he read for the An invitation to return to Cambridge describe his output and methods of mathematical tripos. came in 1930 and Mott accepted a working. His principal posts were at In spite of his mathematical ability, fellowship at Gonville and Caius College. Bristol and Cambridge but he was the Mott was never tempted to be a pure Now married, he found that theoretical “father” of a much larger community, mathematician; physics was his main were rather more respected communicating (without the benefit of e- interest. His first research was undertaken than when he had first entered the mail) with hundreds of scientists, when he was an undergraduate. In the Cavendish. Nevertheless, the prime

Europhysics News 28 28 1997 News Europhysics theoreticians and experimentalists alike, absence of any real suggestions from the interest there was still in PROFILE 85 and 1932 was the Annus Mirabilis of the Europe, particularly in Germany, by the many recall the excitement at these Cavendish, when the was application of to the meetings. Numerous problems and discovered, the split and the understanding of the properties of metals puzzling properties of metals were solved (positive electron), predicted by Dirac, and of the difference between metals and at Bristol during those heady days - was first observed. Stimulated by this semiconductors. The work of Sommer- advancing the subject and, at the same extraordinary atmosphere and a field, Bloch, Peierls and Bethe was time, providing much of benefit to suggestion by Fowler that he write a book especially successful and Mott set about industrial metallurgists. on atomic collisions, Mott collaborated continuing this tradition. He found an In those six pre-war years at Bristol, with to produce a classic enthusiastic supporter in Harry Jones, a Mott did not confine his attention to tome “The Theory of Atomic Collisions”, former research student of Fowler’s, and understanding the physics of metals. He which ran to several editions. together they wrote a famous textbook began his first work on semiconductors, a “Theory of the Properties of Metals and field with which he was to remain attached Bristol Alloys”. Strange as it may now seem to the for many more decades - in fact, well into In 1933, Mott accepted his first Chair, hundreds of -state physicists who his retirement. The initial stimulus came that of Professor of Theoretical Physics at learned their basic quantum mechanics of from Ronald Gurney who had arrived Bristol University. For six years up to the solids from this work, the book was not from Manchester and who, like Mott, had outbreak of the Second World War, he that well received by other theorists, being a remarkable talent for visualizing solids played a significant role in establishing criticised for lack of rigour. What in fact in terms of their constituent and Bristol as one of the foremost centres for we see in this book is mathematics applied electron waves, without the need for solid-state physics. His philosophy, which to the direct interpretation of experi­ detailed mathematics. Together they he never abandoned during the remainder mental data, interspersed with bold wrote another “Mott and X” book (where of his life, was to have theoreticians and initiative ideas which leapfrog the formal in this case X was Gurney) titled experimentalists working closely together. development when the rigorous mathe­ “Electronic Processes in Ionic Crystals”. Together they laid the foundation of the field of colour centres in alkali halides by providing a description of the defects involved in terms of negative ion vacancies. They also worked out the physics behind the photographic process - why falling on a grain of bromide was able to produce a speck of silver - the latent image. In 1940 Mott was awarded the Harker and Driffield medal from the Royal Photographic Society for this work. Four years earlier, at the age of 31, he had been elected a Fellow of The Royal Society. Mott’s personal qualities and human- itarianism were evident after the German occupation of Sudetenland, when there was a movement to rescue children of Physics Department, , 1935 Jewish descent from Czechoslovakia. He Front row, reading left to right looking at the photograph: W. Sucksmlth, H.W.B. Skinner, G.l. Harper, L.C. Jackson, N.F. and his wife, Ruth, with help from his Mott, A.M. Tyndall, S.H. Piper, I. Williams, H.H. Potter, W. Heitler. sister, Joan, housed two young refugees, Second row, reading left to right: ...Huntley, S.E. Williams, K. Fuchs, H. Jones, E.T.S. Appleyard,... Baber, V.E. Cosslett, daughters of a Jewish musician. Lilly and C.F. Powell, J.H. Burrow, K. Worsnop, A.F. Pearce. Use Spielman, who stayed in after Third row, reading left to right: R.W. Gurney, L. Frank, H. Worthy, W.R. Harper, ...Mercer, A.C.B. Lovell, N. Thompson. the war, recall their gratitude to this day. The War and return to Bristol Although many today see the advantages matical treatment fails. This type of During the Second World War, Mott of such a liaison, it has never been easy to pragmatic approach - annoying to some was involved in various defence projects implement such a policy. Mott’s ability to pure theoreticians but beloved by relating to radar, shell fragmentation, the draw theory and experiment together, to experimentalists anxious to have formulae deployment of searchlights and to persuade practitioners on both sides to with which to analyse their data - was to mathematical research on armaments work on common problems - indeed to be seen again during Mott’s career. generally. He succeeded Blackett as blur any distinction between the two From recent accounts of members of scientific adviser to a commanding officer approaches - was one of his hallmarks for the Bristol research group, Mott provided in 1940. When he discovered that the which he will be remembered. At Bristol, a stimulating atmosphere which they number of enemy aircraft destroyed by he deliberately avoided setting up separate recall with fond memories. At mini­ British fire was being overestimated by departments for theorists and experi­ conferences, which he organised with flair quite a large factor, his commander told mentalists, which was the norm elsewhere. and a great sense of timing, participants him that he must not disclose his findings

Great success had been achieved in were incited to perform at their best and until after the war was over! He wrote a 28 News 1997 Europhysics 86 PROFILE paper on why German shells, which were occupy Mott’s time for many years was his That Cambridge was a world apart made of steel with a high carbon content, campaigning against the UK’s develop­ from Bristol became clear on Mott’s fragmented into smaller pieces than the ment of the atomic bomb and his Chair­ introduction at the Cavendish. His first British shells. Apparently, a memorandum manship of the Atomic Scientists’ contact was with the Administrative on this still survives in American Association, whose aims were to explain Secretary of the Department. “What are ordnance laboratories. At about the same the true facts of nuclear energy to a wide your problems?” Mott asked of him. “Well time, the Royal Society awarded him their audience and to investigate proposals for Professor, I did want to talk to you about prestigious for his work its control. the Cavendish cricket match, academic before the war. staff versus assistants”, came the reply. In the last days of the war, Mott was Cavendish Professorship Administrative duties and matters offered chairs in Metallurgy and in In 1953 Mott received an offer of the unrelated to his own research occupied Theoretical Physics at Cambridge, but an Cavendish Chair of Experimental Physics Mott’s attention for a great deal of his time assurance that if he returned to Bristol he at Cambridge, following Sir Lawrence as Cavendish Professor. Reform of the would succeed to Tyndall’s position as Bragg’s retirement one year earlier. He National Sciences Tripos, which permitted head of department attracted him back recalls in his autobiography that he was undergraduates to specialise in physics at there. He assumed the Headship in 1948 not too happy at the thought of leaving an earlier stage of their studies than had and immediately set about appointing new Bristol but felt he could not turn down the been possible previously, was one of his staff. Many of the scientists at Bristol offer to follow in the footsteps of James missions. Another was to change the before the war, e.g. Skinner, Gurney, Harry Clerk Maxwell, Lord Rayleigh, J J. physical sciences curriculum in schools Jones, Heitler and Frolich, took positions Thomson, Ernest Rutherford and through the Nuffield Foundation. In elsewhere and did not return. In their - the previous holders of chairing an advisory committee of that place Mott engaged, amongst others, Jack the prestigious Chair. It might seem body he had to steer a difficult course Mitchell (now occupying a Chair at the strange that the title of Professor of between the protagonists of “learning by University of Virginia) who demonstrated Experimental Physics should be offered to doing”, which involved students being that, in the photographic process, silver a theorist but, apart from Rutherford, encouraged to discover the laws of physics precipitated preferentially along dis­ none of the previous incumbants had been for themselves mainly by experimental locations; Charles Frank, who produced principally experimentalists. One of physics, and ideas based on his own novel theories of dislocations and crystal Mott’s first decisions, taken even before he education, in which the laws of physics are growth and predicted what became known actually took up the post, was to halt plans approached through mathematics, their as Frank-Read sources; Dirk Polder as a for the construction of a linear accelerator validity and beauty becoming clearer senior theorist; Jacques Friedel who for high energy - not when expressed and taught this way. A married Mott’s sister-in-law, before because he was against this type of compromise was called for and to a large returning to France to become leader of research but simply because he did not see extent a successful solution found. As a solid state physics in Paris; and Nicolas that the machine would be able to compete member of the General Board and the Cabrera who worked with Mott on a against similar American installations. Council of Senate, both influential bodies theory of why aluminium and stainless The move back to Cambridge in 1954 of the University, Mott steered through steel do not rust. All these workers by was Mott’s last; he was to remain working important decisions - not least the their individual and joint efforts once there not only up to his official retirement creation of the first science park by again made the Bristol Physics Depart­ in 1971 but for another twenty-five years College which attracted science-based ment into a leading research group. In thereafter. On his appointment, Professor industries to the City. Most universities in 1949 Mott published his first paper on Fred Seitz, the American solid-state the UK now have their own science park. - transitions - a topic which physicist, wrote In 1959 Mott was elected Master of was to retain his interest to the end of his Dear Mott Gonville and Caius College. His prede­ life. This was the period which saw the For several weeks there has been a cessor, Sir , was a invention of the transistor at the Bell consistent report that you have accepted distinguished nuclear physicist who had Telephone Laboratories in America and the appointment as director of the discovered the neutron in 1932 and on Mott took a keen interest in this . Deepest congratul­ whom cooperation between the USA and development. He was later to propose a ations. During our sojourn in Tokyo I the UK in the during theory for what became known as Mott- made bets with our colleagues that this the War virtually depended. Chadwick Schottky barriers in semiconductors. would occur, since it was the most logical had retired prematurely following dissent Administrative appointments slowed solution of the issue. Either my judgement within the College Fellowship and Mott down Mott’s personal research towards the is lucky or very profound.Undoubtedly, this was persuaded that he could heal the end of his Bristol period (i.e. up to 1954). means that you will be very busy during divisions that had developed. His appoint­ He became Dean of the Faculty of Science, the summer and that we will catch only ment, as with Masterships of other served on government committees and fleeting glimpses of you at the various Cambridge colleges, did not require him to was appointed President of the Physical meetings. In any case, we all hope that resign the Cavendish Chair, but the task of Society overseeing its amalgamation with Bristol will manage to hold its level and holding the two positions must have been the . He also took over remain the very fine center of research arduous. Life in Caius tended to centre the Editorship of the Philosophical which you have made it. around the Master’s Lodge, a splendid Magazine and became Chairman of the Sincere regards place to entertain the many distinguished Board of Taylor & Francis, its publisher. visitors. The lodge is situated within the

Europhysics News 28 28 1997 News Europhysics Another activity which continued to College precincts opposite the chapel. PROFILE 87 Parts of it date from the sixteenth century when I described to him work I had been describes how carriers hop between and, in addition to wonderful dining and doing at the University of Illinois on localized states over a distance that is drawing rooms, it contains five bathrooms heavily doped semiconductors, which temperature dependent - so-called and a hatch leading into the college behave as metals when the concentration variable-range hopping. I recall his kitchens through which meals can be of donors or acceptors exceeds a critical making a back-of-the-envelope derivation delivered. C.P. Snow’s novels describe, value, he displayed an interest far beyond of this relationship, a calculation which with much reality, the intrigues and my expectations, particularly as I had not was subsequently done “properly” but politics of college life, particularly in been the first to make such measurements; with essentially the same result. This was, relation to the appointment of the Master. Professor Fritzsche in Chicago and others however, only the beginning of a decade or Mott used to say “He didn’t know the half had undertaken similar and more more of activity on other disordered of it!” After six years of service, he extensive studies. What Mott saw, with his systems - in particular amorphous or resigned from the Mastership having customary and extraordinary insight, was non-crystalline semiconductors - which become tired of the petty wranglings of that the metal-insulator transition in was eventually to lead to the award of the some of the Fellows, just as his prede­ doped semiconductors was intimately Nobel Prize for physics, which he shared cessor and indeed the founder of the related to a theoretical paper by Phil in 1977 with Anderson and van Vleck. College, , had done also. But Anderson on the effect of disorder on the It was an illuminating experience to this was not until after he had reformed electronic states in a solid. Anderson’s see how Mott developed and nurtured the the College’s admission policy (at Cam­ classic paper of 1958 (which he was later to field of amorphous semiconductors. At bridge, students are admitted to the describe as “one that is often quoted but the time, few groups showed any interest University via the colleges), and arranged seldom read”) showed how disorder in them. Although the Xerox Corporation for the first time that Fellows could bring ‘localizes’ electronic states, distinguishing in America had achieved great success in lady guests to dine in Hall. During his these from band-like ‘extended’ states. A with their first “dry” photocopying tenureship, Mott continued his interest in metal-insulator transition occurs when the machine which used amorphous selenium control of nuclear weapons and hosted a Fermi level passes through the energy as the photoreceptor, the physics behind Pugwash Conference in the College, at separating extended from localized states. the process was not understood. In the which he recalled Henry Kissinger arguing The disorder, in the case of heavily doped Soviet Union, Kolomiets had been working with the Russian representatives on their semiconductors, arises from the random on the properties of glasses; in Germany, hostility to China. It was also during his positions of the donors and from the Stuke had a small group and, in the UK, Mastership of Caius that he was awarded electric fields associated with charged Spear was studying transport in thin films his Knighthood. compensating acceptors. Mott was able to of germanium and . What Mott did show that the critical concentration of was to bring together these disparate Non-crystalline solids dopants for the ‘Anderson transition’ was activities using his by now well-tested When the present writer arrived in not dissimilar to that for the ‘Mott methods. Pouncing eagerly on new Cambridge in 1967, Mott was still Head of transition’, the theory of which was based results, he formulated his ideas on these the Cavendish and had once again begun on a screening argument. In considering and communicated them via his familiar to turn his attention seriously to solid- electrical conduction on the insulating handwritten letters to interested parties. state physics. His interest in metal- side of the transition, he formulated what He organised mini-conferences (in the way insulator transitions had never waned and is now known as the Mott T-1/4 law which he had done at Bristol), visited lab-

celebrating the announcement of the joint award of the 1977 physics Nobel prize to Sir Nevill Mott, van Vleck and P.W. Anderson. On the left, Prof. Josef Stuke, head of the Marburg Uni­ versity physics group, with whom Mott collaborated for

over 10 years in the field of amorphous semiconductors 28 News 1997 Europhysics 88 PROFILE oratories for personal discussions, extensively rewritten, appeared in 1979. In for whom Mott was able to find a position suggested PhD thesis topics, and wrote the intervening years he had also written in the Cavendish financed by a college draft papers for wide circulation and “Metal-Insulator Transitions” (1974) and a fellowship and with whom he wrote two comment. Thereby he rapidly became the book for sixth formers “Elementary more books. The field is still too new to father figure of a growing community and Quantum Mechanics” (1972). When the assess Mott’s contributions to the subject the Cavendish became the conduit through Cavendish Laboratory moved from its old but, whatever future scientific judgement which ideas were channelled. During a home in Free School Lane to its present be cast on them, he continued to stimulate visit to America he was introduced to Stan site in West Cambridge, Mott collaborated and inspire colleagues, both young and Ovshinsky who had started a small with Abe Yoffe and Mike Pepper in their old, in his unforgettable manner and by company in Detroit, Energy Conversion respective studies of low-dimensional his wise counsel until well past his Devices (ECD), to exploit the potential of crystals and silicon inversion layers, as ninetieth birthday - an astonishing non-crystalline materials. Ovshinsky had well as with numerous long-stay visitors achievement. developed two forms of electronic switch who were attracted to Cambridge by his Throughout his long career, Mott’s based on chalcogenide glasses - devices late bursts of scientific activity and the energy and enthusiasm pervaded much of which, for the most part, were dismissed interest he showed in their work. twentieth century physics. His appetite for by other companies as being of little new challenges seemed inexhaustible and importance. But Mott thought otherwise his contributions to science will be quoted and attempted to explain their behaviour Final decades well into the next millenium. But what in terms of his new concepts on During the late 1980s, Mott turned his may not be recorded adequately is his conduction in disordered materials. The attention to another major field of influence on the lives and careers of possibility of commercial utilization of scientific discovery - that of high- numerous friends and colleagues whom he amorphous semiconductors (whether it temperature superconductivity. In this befriended, advised and guided with such was to be fulfilled or not was of little endeavour he collaborated principally with great generosity. He will be sorely missed. consequence) had a dramatic influence on Sasha Alexandrov - a Russian theoretician, Prof. E.A. Davies, University of Leicester, UK the growth of the field. The third conference in the series “International Conference on Amorphous and Liquid Semiconductors” (held every other year since 1965) took place in Cambridge and the number of participants was four to five times that of the previous meeting. Mott chaired most of the sessions, giving his own views and interpretation of virtually every paper presented. The concepts of mobility edges, hopping conduction, minimum metallic conductivity, the “8-N Prof. Dr. phil., Dr. h. c. mult. Friedrich Hund rule”, relaxation of the k-selection rule, tail states, dangling bonds etc. were added *4. Februar 1896 t31. März 1997 to the vocabulary of practitioners in the Die Wissenschaft war Mitte seines Lebens. field; it would not be too generous to give Mott credit for introducing most of these Wir trauern um unseren Vater, Großvater und Urgroßvater ideas. Another significant event in the Gerhard und Juliane Hund development of the subject was the Irmgard und Prof. Dr. Dieter Ffirsch discovery by Spear’s group of the way to Dr. Martin und Rita Hund dope amorph-ous silicon - providing a Andreas Hund further boost to worldwide interest and Dr. Erwin und Karola Hund leading to the development of solar cells, Enkel und Urenkel thin-film transistors and other devices now used extensively in the electronics industry. This year (1997) sees the 17th. Charlottenburger Straße 19 (GDA-Wohnstift), 37070 Göttingen Die Trauerfeier findet am Mittwoch, dem 9. April 1997, um 12.00 Uhr conference in the same series mentioned in der großen Kapelle des Stadt. Friedhofs Göttingen-Junkerberg above, the only change being to its name - (OT Weende-Nord) statt. it is now the “International Conference on Amorphous and Microcrystalline Bestattungshaus Benstem, Rote Straße 32.37073 Göttingen. Semiconductors”, but it will still start with the “Mott lecture”. Mott’s work in the above field was not interrupted by his retirement in 1971. The first edition of a book, co-authored with the present writer, “Electronic Processes in Non-Crystalline Materials” was published A Profile of Friedrich Hund will appear in one of the next issues

Europhysics News 28 28 News 1997 Europhysics in that year and a second edition, of Europhysics News.